Pistons in two-stroke engines have compression rings like their four-stroke counterparts. Some two-stroke engines that have ports in the cylinder wall, which are covered and uncovered by a piston, have an additional oil ring in the piston that is below the ports. The oil ring prevents oil that is splashing around for lubrication and cooling from too much access to the ports and being combusted in the combustion chamber or leaving directly through exhaust ports. The oil ring also provides oil dosing, to allow some oil to lubricate the compression rings and the piston skirt. In addition, the oil ring in a two-stroke engine has to seal the pressurized air at the intake ports or the exhaust gas at the exhaust ports from escaping into the crankcase. Such an arrangement leads to a piston or liner that is longer than may desirable for some applications. An alternative is to provide a sealing ring (stationary ring) in a groove in the cylinder wall such as is shown in FIGS. 1a-c. 
In FIG. 1a, a piston 10 having compression rings 12 and 14 is traveling downward within a cylinder wall 20 (only a portion of one side of the wall is shown in the cross section in FIG. 1a) that has ports 22 in the wall to allow flow of gases into the cylinder, if the ports are intake ports, and flow of gases out of the cylinder, if the ports are exhaust ports. A circumferential groove is formed in cylinder wall 20 into which a stationary oil ring 24 is placed. In FIG. 1a, piston 10 is in a position such that compression rings 12 and 14 are above port 22. Thus, the combustion chamber (above piston 10) is sealed off from port 22. Piston 10 is moving downward and at the instant illustrated in FIG. 1b, port 22 is in communication with the combustion chamber. As piston 10 moves downward farther, as illustrated in FIG. 1c, piston 10 completely uncovers port 22.
A view of a piston 370 in a cylinder liner 368 is shown in FIG. 13. Piston 370 has compression rings 374. Cylinder liner 368 has ports 376 and a groove 372 defined there. A stationary ring may be installed in groove 372 to assist with oil and/or gas control.
Cylinder wall 20 in FIGS. 1a-c is shown as one piece. However, stationary oil ring 24, which is a split ring, has a large gap if inserted into cylinder wall 20 and then sprung into the groove in the cylinder liner. In some alternatives, the cylinder wall is formed out of two pieces. In FIG. 1c, a dotted line 26 in cylinder wall 20 indicates a parting line that can be used in a two-piece cylinder liner to facilitate installation of stationary oil ring 24. Problems with two-piece cylinder liners include: increased weight to include a flange and bolts to secure the two pieces together and difficulty in ensuring that the cylinder walls of the two pieces are perfectly aligned and machined properly. A ring system that facilitates installation into a one-piece cylinder liner is desired.